Our long-term goal is to understand mechanisms of tumor progression and metastasis and to develop new approaches for molecularly targeted therapeutics. Metastasis, a multi-step process in which primary tumor cells disseminate to distant sites of the body and form secondary tumors, is the leading cause of cancer deaths and remains one of the most poorly understood pathological processes. New biomarkers of metastatic diseases and useful targets for therapeutic intervention, including proteins, microRNAs, and other non-coding RNAs, continue to be highly sought. We and others have demonstrated the existence of metastasis-promoting and metastasis-suppressing microRNAs. Moreover, we provided proof-of-concept evidence that antagomirs targeting a pro-metastatic microRNA can be efficiently delivered to fast growing tumor cells in vivo and can block metastasis formation by otherwise highly malignant cells. The molecular mechanisms by which these individual microRNAs function in metastatic progression warrant further investigation. In this application: 1) we will identify and characterize the functional targets of metastasis-promoting microRNAs. Combining high- throughput RNA sequencing, computational approach, functional experiments, mechanistic analysis, and patient studies, we will investigate a microRNA target that represents a novel suppressor of metastasis. 2) We will study how two distinct metastasis-regulating microRNAs are connected. Our preliminary data demonstrated that expression of a specific metastasis-promoting microRNA (miR-9) leads to downregulation of the primary transcript but upregulation of the mature form of another pro-metastatic microRNA (miR-10b). Subsequently, we will investigate the mechanisms by which miR-9 regulates miR-10b biogenesis and determine whether miR-10b is required for mediating miR-9-induced metastasis. 3) We will identify and characterize new epithelial-mesenchymal transition (EMT)-regulating microRNAs. By conducting microRNA expression profiling analysis of cells that have undergone EMT, we have recently identified a microRNA signature of EMT, and discovered that two microRNAs in this signature function as EMT inducers. We will determine the mechanisms by which these two microRNAs induce EMT and whether they regulate tumor metastasis and stem cell properties. The knowledge gained from these studies will fundamentally advance our current understanding of how microRNAs and their targets regulate metastasis and EMT and may have important clinical implications.